Whole blood (WB) comprises three major cellular components, red blood cells (RBCS) and leukocytes (also designated as white blood cells, WBCs) as well as thrombocytes (also designated as platelets, PLTs), which represent cellular fragments derived from precursor cells. A major non-cellular component of the blood is the blood plasma.
Whole blood or blood components may be separated and further processed for a variety of uses, particularly for use as transfusion products.
While WBCs represent an essential part of the body's immune system, the presence of WBCs in a number of products, such as for example transfusion products, is undesirable, because WBCs can cause adverse effects in a patient receiving a blood transfusion, like febrile non-hemolytic reactions, graft-versus-host disease, and immunosuppression. Thus, WBCs are routinely separated from whole blood or blood components for transfusion by filtration.
PLTs are involved in the formation of blood clots, and are therefore often removed from whole blood or blood components, in order to avoid clot formation in a blood product.
Furthermore, it may be desirable to remove other, non-cellular substances from whole blood or blood components, and especially from shed blood (instead of “shed blood” also the term “cell salvage blood” is used).
Commercially available filters for blood filtration and for the depletion of WBCs and/or PLTs from whole blood or blood components are made using membrane technology, fiber technology, or a combination thereof.
Fibers suitable for blood filtration are widely commercially available. Usually, nonwoven fibers produced with different methods such as spunbonding or melt blowing are used in blood filtration applications.
While spunbond fibers typically have a fiber diameter that is at least 20 μm or larger, melt blown fibers may have lower diameters of less than 20 μm. Typical diameters of melt blown fibers are 1 to 3 μm.
The fine melt blown fibers offer the advantage of a high surface area due to their small diameter. Nonwoven melt blown fibers are well suited for the removal of WBCs from whole blood or blood components. The higher surface area of melt blown fibers in comparison to, e.g., spunbond fibers allows for an improved adhesion of WBCs, and thus for a more efficient removal of these cells from the whole blood or blood component to be filtered.
However, a problem that is typically encountered during the filtration of whole blood or blood components is the formation of a so called “gel” or “biofilm”. During filtration, this biofilm, which partially consists of activated platelets, plasma factors and cellular micro-aggregates, is successively deposited on the first filtering layers of a blood filter, and thereby causes a successive reduction in filtration flow that eventually can result in the complete blockage of the filter.
In the prior art, blood filters comprising a pre-filter portion in addition to a main filter portion designated, e.g. for the removal of WBCs from whole blood or blood components, have been used to protect the main filter portion from filter blockage. Often, pre-filters are made of spunbond fibers, because these fibers typically have a comparably large diameter of 20 μm or more, and are therefore better suited for pre-filtration applications than the finer melt blown fibers.
However, the pre-filters used in the prior art do not completely avoid the occurrence of reduced filtration flow or even clogging of the main filter portion, because a part of the biofilm is not efficiently entrapped in the pre-filter and thus penetrates into the main filter portion, which usually, due to its structure, e.g. due to a smaller pore size, is much more prone to blockages caused by the deposition of biofilm.
Attempts have been made to select pre-filtering materials with optimized surface chemistry and density, in order to achieve a better adhesion of the biofilm in the pre-filter designed to entrap the biofilm without blocking blood flow. However, this approach has the shortcoming that it results in increased blood loss, and thus loss of desired filtration product, during filtration, caused by the pre-filter structure.
Often, pre-filters are also constructed so as to provide an enlarged surface by means of a reduced fiber size in order to achieve a better distribution of the biofilm on the pre-filter surface. However, a considerable disadvantage to this approach likewise is the consequent higher loss of blood that is caused by the enlarged pre-filter surface.
Consequently there is a need for an improved blood filter for the filtration of whole blood and blood components that is less prone to flow-reduction or clogging of the filter associated with biofilm formation.